Kinda Weird Electrical Issue... battery related?

Okay, so I try to start my 93 and nothing, no click, no sound. Battery shows 12.66 Volts.
Hooked it up to an RSX and jumper cables and started it, nothing.
Took the battery to Pep Boys to have tested. They said it was fine and it just needed a charge. They charged it. Put it back in and still nothing. The interior light was on when I first hopped in the car suggesting I had electrical power but as soon as I turned the key I heard a pop like a fuse blowing and BAM, all power goes out again.
I’ve had this car 13 years and I’ve seen many problems, but not like this.

Please help.

Thanks!

Check the connections.

Thanks!!!
Loose battery cable. WTF!